Thrift: Python installation.

Thrift: Python installation.

Summary:
Added the configure script variable PY_PREFIX which serves the same purpose
for Python as --prefix does for C/C++.  We chose to do this because Python
has different conventions for where to install libraries.

Reviewed By: mcslee

Test Plan:
Full clean builds and installs of Thrift from the Thrift root,
with and without PY_PREFIX.  Watched output of make install.

Ran configure from lib/py without PY_PREFIX, with PY_PREFIX and with
PY_PREFIX in the environment.  Checked config.status for each.

Revert Plan: okay

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665248 13f79535-47bb-0310-9956-ffa450edef68